Updated

A Drug Enforcement Administration agent and a suspect were left dead and two law enforcement officers were injured Monday during a shooting inside an Amtrak train stopped at an Arizona station, authorities said. 

A second suspect was in custody, Tucson Police Chief Chris Magnus told reporters. Magnus said the Counter Narcotics Alliance, a task force comprised of local and federal law enforcement agencies, boarded a New Orleans-bound Amtrak train stopped at a station in downtown Tucson around 8 a.m. to do a routine check for illegal guns, drugs, money, and other items. 

The law enforcement officers encountered two people on the second level of the double-decker train. While they were speaking with one person, a second opened fire, Magnus said. A DEA agent was shot and died and another was injured. 

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A Tucson Police Department officer walks with his weapon near the scene of a shooting aboard an Amtrak train in downtown Tucson, Ariz., on Monday where a DEA agent and suspect were left dead. (Mamta Popat/Arizona Daily Star via AP)

A Tucson police officer on the platform heard the shots ring out and ran into the train, the chief said. 

"As he entered into the train, he was also shot," Magnus said. 

The suspected shooter then barricaded himself inside a train bathroom after exchanging gunfire with officers, Magnus said. Authorities later determined that the suspect was dead. Magnus did not know whether agents shot him or he took his own life. 

The seriously injured DEA agent and Tucson police officer were taken to a hospital. The police officer was in stable condition and is being held overnight for observation, Magnus said. 

"I also want to reflect on the really heroic actions of the officers at the scene who literally ran towards the danger into the car where there was an active shooting situation going on," Magnus said. 

Arizona Gov. Doug Ducey, a Republican, expressed his condolences to the law enforcement officers in a tweet. 

"My heartfelt condolences are with the loved ones and colleagues of the DEA agent who lost his life today in Tucson," he wrote. "Two other law enforcement officers are injured in the hospital. Our prayers are with them, their families and the law enforcement community."

Anne Milgram, administrator for the DEA, said the federal agency was "heartbroken" over Monday's events. 

"Tragically, this morning, two DEA special agents and a DEA task force officer from the Tucson Police Department were shot during a law enforcement operation in Tucson, Arizona. One DEA special agent died as a result of injuries sustained during the shooting. A second DEA special agent is in critical condition. The DEA task force officer is in stable condition. We at the DEA are heartbroken by today’s events and ask that you keep the families of the agents and task force officer in your thoughts and prayers."

The Sunset Limited, Train 2, was traveling from Los Angeles to New Orleans, Amtrak spokesman Jason Abrams told Fox News. The 137 passengers and 11 crew members on board were uninjured and evacuated from the station. 

Amtrak is working with local and federal authorities, he added.

Security footage at the station posted online shows a train arriving at the station while other footage shows some of the shooting. 

Multiple shots can be heard from inside a train before a man, who appears to be a security officer with a dog, boards in the middle of the second-to-last car through an open door. Two bystanders back away and then run past a baggage cart, joining four others as they usher each other into the last car and the door slides shut.

KOLD-TV previously reported an officer had been injured from gunfire. A source told the news outlet the injured officer was taken to a hospital.

A Tucson Police Department SWAT truck is parked near the last two cars of an Amtrak train in downtown Tucson, Ariz., on Monday. One person is in custody after someone opened fire Monday aboard an Amtrak train parked at the station in the city's downtown. (Mamta Popat/Arizona Daily Star via AP)

A witness said two men approached another on a train saying they wanted to speak with him. As they left the train, shots were fired, KOLD reported. 

Evan Courtney was in a lounge car when people suddenly came running in yelling: "Shots fired!"

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"I grabbed my backpack and ran," he told The Associated Press via Twitter direct messaging.

He later tweeted photos of officers embracing each other. 

The Associated Press contributed to this report.